Introduction to Petrology

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INTRODUCTION

TO PETROLOGY

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Petrology

•

• Petrology is the study of rocksPetrology is the study of rocks

•

• Petrology deals with Petrology deals with the origin, occurrence,the origin, occurrence,

mineral composition, chemical and

mineral composition, chemical and physicalphysical propertie

properties of s of rocksrocks

•

• Rocks are natural massive aggregates ofRocks are natural massive aggregates of

minerals, forming the crust of the Earth minerals, forming the crust of the Earth

•

• Petrology is further Petrology is further sub-divided intosub-divided into

Petrography

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Geological Classification of Rocks

•

• Igneous rocksIgneous rocks

•

• Sedimentary rocksSedimentary rocks

•

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The Rock Cycle

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Texture & Structure

o

o Texture Texture refers to the mutual relationship of therefers to the mutual relationship of the

different mineralogical consti

different mineralogical constituents in a tuents in a rockrock

o

o Structure Structure refers to the large scale features orrefers to the large scale features or

field characteristics of the rocks field characteristics of the rocks

Importance: Importance: •

• contribute to the strength of the rockcontribute to the strength of the rock

•

• act as act as a distinguishing featura distinguishing featuree

reveal the mode of origin of the rock reveal the mode of origin of the rock

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(a) Igneous rocks

•

• They are formed due to the cooling ofThey are formed due to the cooling of

magma/lava magma/lava

•

• They are called primary rocks, as they are theThey are called primary rocks, as they are the

first formed rocks that made up

first formed rocks that made up the primordialthe primordial

Earth’s crust Earth’s crust

•

• The basic classification of igneous rocks is intoThe basic classification of igneous rocks is into

extrusive and intrusive igneous rocks extrusive and intrusive igneous rocks

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Classification based on the depth

of

formation

o

o Volcanic rocksVolcanic rocks –– formed on the surface of theformed on the surface of the

Earth Earth

o

o Plutonic rocksPlutonic rocks –– formed at considerable depthsformed at considerable depths o

o Hypabyssal rocksHypabyssal rocks –– formed at intermediateformed at intermediate

depths (<2km) depths (<2km)

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Granite, Intrusive

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Basalt, extrusive Basalt, extrusive

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Average mineralogical composition of

Igneous rocks

Sl Sl No No Mineral Mineral (%)(%) i i Feldspars Feldspars 59.559.5 ii

ii Pyroxenes Pyroxenes and and Amphiboles Amphiboles 16.816.8 iii

iii Quartz Quartz 12.012.0

iv iv Biotite Biotite 3.83.8 v v Titanium Titanium 1.51.5 vi vi Apatite Apatite 0.60.6 vii

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Factors defining textures of Igneous

rocks

•

• Degree of Degree of CrystallizatiCrystallizationon

1.

1. HoHololocrcrysystatallllininee 2.

2. HoHololohhyayalilinene 3.

3. MeMerorocrcrysystatallllininee

•

• GranularityGranularity

1.

1. CoCoararsese-g-grarainineded 2.

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Types of Textures

1

1.. EEqquuiiggrraannuullaarr 2

2.. IIneneqquiuiggraranunulalarr 1 1.. PPoorrpphhyyrriittiicc 2 2.. PPooiikklliittiicc 3 3.. DDiirreeccttiivvee 4 4.. IInntteerrggrroowwtthh 5 5.. IInntteerrggrraannuullaarr

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Equigranular texture -Granite Equigranular texture -Granite

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Porphyritic texture - Andesite Porphyritic texture - Andesite

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Poiklitic texture

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Intergrowth in Lunar Granite

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Structures of Igneous rocks

•

• Structures due to mobility of magma/lavaStructures due to mobility of magma/lava

•

• Structures due to cooling of magmaStructures due to cooling of magma

•

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Structures due to mobility of magma

1.

1. FloFlow w strstructuctureures s 2. Pil2. Pillow low strstructuctureuress 3.

3. Ropy Ropy and and blocky blocky lava lava 4. 4. SpheruliSpherulitic tic structuresstructures 5.

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Structures due to cooling of magma

1.

1. JoJoinintiting ng ststruructctururee 2

2.. RRiifft t aannd d ggrraaiinn 3.

3. VeVesisicuculalar sr strtrucuctuturere 4.

4. MiMiararololititic ic ststruructctururee

Miscellaneous structures

1. Reaction structure 1. Reaction structure 2. Xenolithic structure 2. Xenolithic structure

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Forms of Igneous rocks

Refers to the shape in which cooled igneous Refers to the shape in which cooled igneous masses occur in nature:

masses occur in nature: Form is decided by:

Form is decided by:

o

o Structural disposition of host rockStructural disposition of host rock o

o Viscosity and composition of magma or lavaViscosity and composition of magma or lava

Types: Types: o o ConcordantConcordant o o DiscordantDiscordant

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1. Concordant bodies

• • SillsSills • • PhacolithsPhacoliths • • LopolithsLopoliths • • LaccolithsLaccoliths

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Sill

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Salisbury Crags, Edinburgh, Scotland, a sill exposed Salisbury Crags, Edinburgh, Scotland, a sill exposed during the ice ages during the ice ages

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Phacolith Phacolith

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Lopolith Lopolith

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Laccolith Laccolith

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Laccolith exposed by erosion of overlying Laccolith exposed by erosion of overlying strata in Montana strata in Montana

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2. Discordant bodies

•

• Dykes/dikesDykes/dikes

•

• Volcanic necksVolcanic necks

•

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Dike on the Baranof Cross-Island trail, Alaska Dike on the Baranof Cross-Island trail, Alaska

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Magmatic Intrusions Magmatic Intrusions

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(b) Sedimentary rocks

•

• They are formed by the accumulation,They are formed by the accumulation,

compaction and consolidation of sediments compaction and consolidation of sediments

•

• They are secondary rocks, derived from theThey are secondary rocks, derived from the

sediments produced by the weathering of sediments produced by the weathering of pre-existing rocks

existing rocks

•

• The accumulation and compaction of theseThe accumulation and compaction of these

sediments usually take place in the

sediments usually take place in the presence ofpresence of water

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Environment of Formation

•

• Continental faciesContinental facies

•

• Transitional faciesTransitional facies

•

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Varying Mineralogical Composition

Factors influencing

Factors influencing mineralogical composition:mineralogical composition:

o

o Nature of Nature of gathering groundgathering ground o

o Duration of transportDuration of transport o

o Mixing up of sedimentsMixing up of sediments o

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Textures of Sedimentary Rocks

Textures are determined by: Textures are determined by: 1

1.. OOririgigin n of of ggrraiainsns

o

o Clastic and non-clastic texturesClastic and non-clastic textures

2

2.. SSiizze e of of ggrraaiinnss

o

o Coarse-graineCoarse-grained d - - avg avg grain grain size size >5mm>5mm o

o Medium-graineMedium-grained d - avg grain size - avg grain size b/w 5 & 1mmb/w 5 & 1mm o

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3. Shapes of grains 3. Shapes of grains

o

o Rounded, sub-rounded, angular & sub-angularRounded, sub-rounded, angular & sub-angular

4. Packing of grains 4. Packing of grains

o

o Open-packed (porous) and densely packedOpen-packed (porous) and densely packed

5

5.. FaFabrbric ic of of ggrraiainsns

o

o Described in terms of Described in terms of orientation of longerorientation of longer

axes of grains axes of grains

6. Crystallization trend 6. Crystallization trend

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1. Mechanical structures

i i.. SSttrraattiiffiiccaattiioonn i iii.. LLaammiinnaattiioonn ii

iiii.. CrCrososs bes beddddiningg iv

iv.. GrGradadeed d bbeeddddiningg v

v.. MMuud d ccrraacckkss v

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Stratification in the Grand Canyon Stratification in the Grand Canyon

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Lamination in Travertine

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Cross-bedd

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Mud cracks Mud cracks

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Rain prints

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Ripple marks

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2. Chemical structures

1.

1. CoConcncreretitiononarary sy strtrucuctutureress 2.

2. NoNoddulular sar sttrurucctuturree 3.

3. GeGeodode e sstrtrucuctuturree

3. Organic

structure

s

1.

1. FoFossssililififererouous s ststruructctururee 2.

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Concretionary structure

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Geode structure

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Classification on the basis of mode of

formation

o

o Clastic or Mechanically formed rocksClastic or Mechanically formed rocks o

o Non-clastic rocksNon-clastic rocks o

o Chemically formed rocksChemically formed rocks o

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1. Clastic rocks

Type

Type Sub-division Sub-division SizeSize

Gravels Gravels Boulders Boulders > > 256 256 mmmm Cobbles Cobbles 16-256 16-256 mmmm Pebbles Pebbles 2-16 2-16 mmmm Sands Sands Coarse

Coarse sands sands 0.5-2 0.5-2 mmmm Medium

Medium sands sands 0.25-0.5 mm0.25-0.5 mm Fine

Fine sands sands 0.0625-0.25 0.0625-0.25 mmmm Silts 1/256

Silts 1/256 –– _{1/16 mm}_{1/16 mm}

Clay

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2. Non-clastic rocks

i i.. CChheemmiiccaalllly y ffoorrmmeed d rroocckkss 1 1.. SSiilliicceeoouus ds deeppoossiittss 2 2.. CCaarrbboonnaatte de deeppoossiittss 3

3.. FeFerrrurugiginnouous s dedeppososititss 4

4.. PhPhososphphaatitic c dedeppososititss 5

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ii.

ii. Organic Organic depositsdeposits 1

1.. CCaarrbboonnaatte e rroocckkss 2

2.. CCaarrbboonnaacceeoouus rs roocckkss 3

3.. PhPhososphphaatitic c dedeppososititss 4

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(c) Metamorphic rocks

•

• Metamorphism Metamorphism means change of formmeans change of form

•

• Metamorphism relates to the processesMetamorphism relates to the processes

responsibl

responsible for the changes in a e for the changes in a rock under therock under the influence of temperature, pressure and

influence of temperature, pressure and chemically active fluids

chemically active fluids

•

• Metamorphic rocks are formed from pre-Metamorphic rocks are formed from

pre-existing rocks, under the influence of the

existing rocks, under the influence of the aboveabove

factors factors

(54)

Quartzite, Prospect Mountain, Wheeler Peak, Quartzite, Prospect Mountain, Wheeler Peak, Nevada, The U.S.A Nevada, The U.S.A

(55)

Terminology

•

• Ortho-metamorOrtho-metamorphic rocks - phic rocks - formed fromformed from

igneous rocks igneous rocks

•

• Para-metamorphiPara-metamorphic c rocksrocks –– formed fromformed from

sedimentary rocks sedimentary rocks

(56)

Metamorphic Agents

• • TemperatureTemperature • • PressurePressure •

(57)

1. Temperature

•

• Minerals are normally stable at Minerals are normally stable at temperaturetemperaturess

below 200

below 200°° CC

•

• Sources of heat for metamorphism:Sources of heat for metamorphism:

–

– The internal heatThe internal heat

–

– The magmatic heatThe magmatic heat

•

• Metamorphic changes take place betweenMetamorphic changes take place between

300

(58)

2. Pressure

•

• Pressure causing metamorphisPressure causing metamorphism is m is of twoof two

types: types:

–

– Uniform pressure (due to over-burden),Uniform pressure (due to over-burden),

acts vertically downwards acts vertically downwards

–

– Directed pressure (from orogenic Directed pressure (from orogenic activity),activity),

can act in any direction can act in any direction

(59)

3. Chemically active fluids

•

• E.g.: water/steam, COE.g.: water/steam, CO22, hydrofluoric acid etc., hydrofluoric acid etc.

•

• These fluids act as carriers of These fluids act as carriers of chemicalchemical

components that drive the chemical reactions components that drive the chemical reactions with the minerals

with the minerals

•

• The pore fluids undergo expansion, with rise inThe pore fluids undergo expansion, with rise in

temperature temperature

•

• Fluids present around rocks may react with theFluids present around rocks may react with the

minerals within them, at

(60)

Types of Metamorphism

1.

1. ThTherermamal mel metatamomorprphihismsm 2.

2. DyDynanamimic mec metatamomorprphihismsm 3.

3. DynDynamoamo-th-therermalmal/Re/Regiogional menal metamtamorporphishismm 4

(61)

(a) Thermal metamorphism

Refers to all

Refers to all metamorphic processmetamorphic processes ines in which heat plays a predominant role.

which heat plays a predominant role.

o

o Contact metamorphismContact metamorphism o

o Pyro Pyro metamorphismetamorphismm o

(62)

(b) Dynamic

metamor

phism

o

o Pressure/stress plays the predominant rolePressure/stress plays the predominant role o

o Also known as cataclastic, mechanical orAlso known as cataclastic, mechanical or

dislocation metamorphism dislocation metamorphism

o

o Stress is more effective at higher levels ofStress is more effective at higher levels of

the crust, where rocks are rigid and brittle the crust, where rocks are rigid and brittle

o

o Pressure causes movement of and interactionPressure causes movement of and interaction

between rocks, resulting in

between rocks, resulting in their mechanicaltheir mechanical breakdown

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(c) Dynamo-thermal metamorphism

•

• Also known as Also known as Regional MetamorpRegional Metamorphismhism

•

• It refers to metamorphism under the combinedIt refers to metamorphism under the combined

action of all

action of all the three agentsthe three agents

•

• Most prevalent of all Most prevalent of all metamorphic procesmetamorphic processesses

•

• Such conditions were available during theSuch conditions were available during the

mountain building activity, in the history of the mountain building activity, in the history of the earth

(64)

(d)

Metasomat

ism

Refers to the formation of new

Refers to the formation of new minerals byminerals by the chemical replacement of the existing ones, the chemical replacement of the existing ones, under the influence of

under the influence of chemically active fluidschemically active fluids The chemically active fluid may be provided:

The chemically active fluid may be provided:

o

o from within the rock (mineral metasomatism)from within the rock (mineral metasomatism) o

(65)

Types of Metasomatism

• • HydrothermalHydrothermal • • PneumatolyticPneumatolytic • • AdditiveAdditive • • ExpulsiveExpulsive

(66)

Effects of Metamorphism

•

• RecrystallizationRecrystallization

•

• Rock flowageRock flowage

•

• GranulationGranulation

•

(67)

Examples of Metamorphic changes

•

• Igneous rocksIgneous rocks

Granites undergo dynamic metamorphism, to Granites undergo dynamic metamorphism, to form crush breccia

form crush breccia

•

• Sedimentary rocksSedimentary rocks

Pure limestone,

Pure limestone, re-crystalre-crystallizes underlizes under

conditions of contact metamorphism, to conditions of contact metamorphism, to

(68)

Metamorphic Grades

•

• Represents the extent to which an original rockRepresents the extent to which an original rock

has been changed by metamorphism. has been changed by metamorphism. •

• The grades are indicated by the presence of aThe grades are indicated by the presence of a

set of index minerals set of index minerals

•

• Low gradeLow grade

•

• Medium gradeMedium grade

•

(69)

Metamorphic zones

Indicate the depth wise extension of Indicate the depth wise extension of particular grades of

particular grades of metamorphism:metamorphism: 1.

1. ThThe Epe Epizizonone (te (temempeperaratuture < 3re < 30000°° C)C)

2.

2. ThThe Me Mesesozoozone ne (te(tempemperatrature ure b/w b/w 300300°° - 500- 500°° C)C)

3

(70)

Mineralogical composition of

Metamorphic rocks

Depends upon: Depends upon: o

o The composition of the parent rockThe composition of the parent rock o

o Type and degree of metamorphismType and degree of metamorphism

Types of minerals formed: Types of minerals formed:

o

o Stress mineralsStress minerals o

(71)

Textures of Metamorphic rocks

•

• CrystalloblastiCrystalloblastic c texturetexture

•

(72)

Structures of Metamorphic rocks

1.

1. CaCatataclclasastitic stc struructctururee 2.

2. ScSchihiststosose se strtrucuctuturere 3.

3. GnGneieissssosose se strtrucuctuturere 4.

4. MaMacuculolose se ststruructctururee 5.

(73)

A.

A. _{Schistose structure}_{Schistose structure} B.

B. _{Granulose structure}_{Granulose structure}

A. Cataclastic structure

B. Maculose structure

Structures of Metamorphic rocks Structures of Metamorphic rocks

(74)

Large scale structural features

•

• Rock cleavageRock cleavage

–

– Flow cleavageFlow cleavage

–

– Fracture cleavageFracture cleavage

•

• SchistositySchistosity

•

(75)

Gneiss, a foliated Gneiss, a foliated metamorphic rock metamorphic rock

(76)

Classification of Metamorphic rocks

•

• Foliated rocksFoliated rocks –– rocks that show parallelism inrocks that show parallelism in

their mineralogical and structural

their mineralogical and structural constitutionconstitution e.g. slates, phyllites

e.g. slates, phyllites

•

• Non-foliated rocksNon-foliated rocks –– characterized by thecharacterized by the

absence of foliation absence of foliation

(77)

Reference

•

• Parbin Singh,Parbin Singh, Engineering and General Geology Engineering and General Geology , S K, S K

Kataria & Sons

•

• Chenna Kesavulu, N,Chenna Kesavulu, N, Textbook of Engineering Geology Textbook of Engineering Geology ,,

MacMillan India

•

• Reddy, D V,Reddy, D V, Engineering Geology Engineering Geology , Vikas Publishing, Vikas Publishing

House

•

• Garg, S K,Garg, S K, Physical and Engineering Geology Physical and Engineering Geology , Khanna, Khanna

Publishers

Thompson, G R and J Turk,